Vehicular airbags play an important role in restraining a seat occupant in vehicular crash situations. However, and in some applications, it may be desirable to suppress the deployment of an air bag. In addition, the deployment of an air bag corresponding to an unoccupied seat represents an unnecessary repair expense.
Vehicles are provided with seat restraint systems such as a seat belt in order to restrain an occupant in a seat of the vehicle. In some vehicles, the seat restraint system may be a lap belt, a shoulder belt, or both. Occasionally, the lap belt and shoulder belt are connected together at one end. The seat restraint system includes a latch plate at the connected end. The seat restraint system also includes a buckle connected at one end by webbing or the like to the vehicle structure. The buckle receives the latch plate to be buckled together. When the buckle and latch plate are buckled together, the seat restraint system restrains movement of the occupant to help protect the occupant during a collision.
Some inflatable restraint systems want input information as to the occupancy of the vehicle seat. Deployment of the inflatable restraint may partially depend on information supplied by sensors in the seat, such as a sensor that would determine the weight of an object in the seat.
When a child seat is placed in the seat and cinched down, the sensors may need a way to distinguish between a large mass and a child seat. Typically, when a child seat is used, there will be high tension in the seat restraint system. Comfort studies have typically shown that a human occupant would not wear their seat restraint that tightly. Readings on seat restraint tension can help to decide the deployment characteristics of the inflatable restraint.
Thus, it may be desirable under certain conditions to provide information to a control module to help to determine the difference between a child seat or an occupant.
Typically, one of two designs are employed to determine seat restraint tension. In one, a compliant design measures the deflection and uses a spring constant to determine the tension. However, this design requires added space for deflection and is impacted by mechanical and electrical/magnetic noises.
The other uses a non-compliant design that directly measures the strain to determine the stress or force information acting on the seat restraint. Typically, a load cell is employed that is more accurate and no observable deflection exists. However, the load cell is required to sustain an overload force (e.g., 18,000N) of about one hundred times a normal operation maximum force (e.g., 180N) without damage to the load cell. In other words, the load cell is required to tolerate a maximum overload force without driving the load cell out off its elastic range. In these load ranges, current load cells only use one percent of the maximum load range which results in low signal levels and poor resolution.
Accordingly, a seat belt tension sensor is desired to generate signal levels that provide better resolution while providing overload protection to prevent damage to the sensor.